Lasers of this description, known in the art under the designation TEA, have the inherent drawback that the discharge becomes unstable with internal pressures exceeding a few torrs, owing to the transformation of the glow into a localized arc interfering with the pumping action of the trigger circuit. Various attempts have been made to solve this problem, these solutions generally involving the pre-ionization of the gaseous laser atmosphere by ultraviolet radiation. Such radiation may be periodically emitted by an ancillary source, activated between pulses applied to the laser electrodes, yet this technique tends to give rise in a sealed laser to a progressively increasing oxygen content reducing its service life; see, for example, an article by B. Norris and A.L.S. Smith titled "Sealed Mini-TEA CO.sub.2 Laser"published on page 75D of the Proceedings of the 1979 Conference on Laser Engineering and Applications. Also known is the Lamberton-Pearson method which utilizes a corona-type discharge between an ancillary wire electrode and one of the main laser electrodes closely adjoining same. The wire may be incorporated in a dielectric board, e.g. as described in two articles by H.M. von Bergmann, V. Hasson and D.J. Brink on pages 511-516 and 524-529 of the Proceedings of the 1979 International Conference on Lasers. The presence of such a board, however, may cause some contamination by organic substances. A modification of that method, described and shown in U.S. Pat. Nos. 4,292,600 and 4,367,553, employs a pointed ancillary electrode in lieu of a wire.
The Lamberton-Pearson method and its variations avoid the need for a separate UV source but, aside from structural complexity, have the disadvantage of unsymmetrical ionization which unfavorably affects the resulting laser beam. This disadvantage is avoided in a structure described on pages 106-115 of a treatise by G.J. Ernst, PhD, published 1977 by the University of Technology of Enschede, Netherlands. The latter structure comprises a strip line of the Blumlein type with a pair of copper plates separated by a layer of Mylar. One copper plate, which is grounded, externally adjoins one of two glass sidewalls of a prismatic housing bounding the laser cavity while the other is energized to set up an oscillatory field across the electrodes which causes a corona discharge passing from one electrode to the other along the mutually parallel housing walls separating same. The UV radiation caused by this discharge ionizes the gas in the cavity.